1. Field of the Invention
The present invention relates to a signal generating scheme, and more particularly, to a signal generating apparatus and related method for performing a phase modulation upon a pulse width modulation signal.
2. Description of the Prior Art
A digital power amplifier (also called a class-D power amplifier) has an advantage of high efficiency and therefore is generally applied to a variety of audio processing equipments. A digital audio power amplifier utilizes a pulse width modulation (PWM) circuit to generate a pulse width modulation signal according to a digital input signal, for controlling the operation of a power output stage.
Please refer to FIG. 1. FIG. 1 is a spectrum diagram showing a pulse width modulation signal Spulse outputted by a prior art PWM circuit. In the time domain, a period of the pulse width modulation signal Spulse equals 1/ft, where ft is a carrier frequency of the pulse width modulation signal Spulse. The PWM circuit performs a PWM operation upon each cycle of the pulse width modulation signal Spulse, so each pulse width of the pulse width modulation signal Spulse may be different from each other. However, in this spectrum diagram, the pulse width modulation signal Spulse comprises three components. The first component includes the original data signal pulses 11 that arise in base band. The second component includes carrier frequencies 12, i.e. the frequencies ft, 2*ft, and 3*ft. The third component includes noise components 13 (i.e. oblique lines) shown in FIG. 1. As a result, by observing FIG. 1, it can be seen that a problem caused by serious electromagnetic interference (EMI) is introduced into the noise components 13 and the carrier frequencies 12.